Recording and reproducing of sound waves



April 2, 1929. Q H, FE1-TER 1,707,260

RECORDING AND REPRODUCINGI 0F SOUND WAVES Passes 700'- 500 V fein/'aduana' Load Spent/cel' mVENToR. C. Zelf' A TTORNEY Patented Apr. y2, 1929.

UNITED STATES PATENT ori-"ICE,

CHARLES H. FETTER, F MILLBURN, NEWIJ'ERSEY, ASSIGNOR T0 AMERICAN TELE- PHONE. AND TELEGRAPH COMIANY, A CORPORATION OF NEW YORK.

RECORDING AND REPRODUCING 0F SOUND WAVES.

Application filed November 10, 1927. Serial No. 232,450.

This invention relates to the recording and reproducing of sound waves, and more particularly to methods and means for recordjing sound waves electrically on a record and electrically reproducing the waves with a phonograph.

In recording speech and music involving a frequency band of 100 to 5,000 cycles on a phonograph record and` reproducing the 1o speech and music from the record, a difficulty in obtaining reproduction of satisfactory y quality is encountered at the reproduction end of thesystem, where noise is introduced because lof the physical contact between the needle and the record. The principal object of this invention is to eliminate this noise or needle scratch.`

The advantage found in the attainment of this object, and various other advantages of the invention, will appear in the following description.

It has been found experimentally that most of the noise known as needle scratch occurs below a frequenc of approximately 5,000

2,5 cycles. In genera the method employed by the applicant for avoiding vsuch noise consistsof cutting the record with a side-band signal rather than with the original voice signal and then demodulating the output at the reproduction end of the system to restore the original voice waves.

The invention will be more clearly understood when the following detailed descrip- ,tion of two desirable embodiments thereof, and the recording and reproduction methods involved, is read with reference to the accompanying drawing. The drawing shows schematically the apparatus and circuit arrangements by which the "applicants object is accomplished. Figure 1 of the drawing shows the recording end of the system; Fig. 2 shows the reproduction end; and Fig. 3 shows an alternative arrangement for the reproduction end.` i

With reference to the details of Figs. 1 and 2 of the drawing, there istransmitted from a transmitter or othersignal source signals covering a frequency range of 100 to 5,000 cycles, for instance. These signals are passed through an amplifier A,l and into a modulator, where they are caused to modulate a carrier of 10,000 cycles froman oscillator Ol. The carrier modulated with the signal is then passed into a filter F1 which passesonly the side-band of frequencies from 5,000 cycles to 9,900 cycles. This side-band may then be amplified in an amplifier A2, and by means of any suitable record-cutting device the record is cut at these side-band frequencies.

At the reproduction end of the system an electrical phonograph and reproducer sets up in the circuit the side-band frequencies of 5,000 to 9,900 cycles, at which the record was cut. In the reproduction, however, there may. be introduced the needle scratchy frequencies which, as stated above, lie, for thev most part, below 5,000 cycles. In order to eliminate these noise frequencies, a filter F2 is introduced in the circuit, this filter passing only i the frequencies from 5,000 to 9,900 cycles. These side-band frequencies are then beaten by a 10,000 cycle frequency from an oscillator O2v in the demodulator to restore the original signal. A filter F 3 may be introduced which passes only the frequencies from 100 to 5,000 cycles, thus eliminating the carrier and any other undesirable frequencies which may be present. The resultant waves may then be passed' through an amplifier A3 and on to a loud speaking receiver.

Instead of being supplied bythe oscillator CFI O2 of Fig. 2, the carrier may be supplied at Y the reproducingend as indicated in Fig. 3.

A sinUle trial groove may be cut in the record of 'the phonograph and a 'second reproducer (upper) caused to run in this groove to give the proper demodulating frequency.

At the output of the lower reproducer, which gives the side-band, a filter F passes the frequencies from 5,000 to 9,900 cycles. The filter F in the output of the upper reproducer passes frequencies from 5,000 to'10,100 cycles, for instance, thus passing the carrier. These two filters serve to eliminate the needle scratch frequencies. The portions of the reproducer outputs which pass these filters are then passed into a demodulator, the balance of the circuit corresponding to that shown in Fig. 2..

While the invention has been specifically described for the purpose of illustration, it is to be understood that its scope is not limited -by such description but is defined in the approduce a side-band, recording the side-band, reproducing the side-band, eliminating after reproduction all frequencies outside the range of the side-band, and beating the side- 5 band with a high frequency equal to the carrier to restore the original waves.

2'. The method of recording and reproducing sound waves which consists in modulat ing a carrier wave with the Original waves t0 produce aside-band having a lower limit of approximately 5,000 cycles per second, recording the side-band, reproducinl the sideband, and beating the side-band w1th a high frequency equal to the carrier to restore the original waves.

3. The method of recording and reproducing sound waves varying in frequency over a band of approximately 100 to 5,000 cycles per second which consists in modulating a carrier wave with the original waves to produce a side-band having a lower limit of approximately 5,000 cycles per second, separating said side-band from the carrier, recording said side-band, reproducing said side-band, eliminating after reproduction all frequencies outside the range of said side-band, and beating said side-band with a high frequency yequal to the carrier to restore the original waves.

4. In a system for recording and reproducing sound waves, means for combining the original waves with a carrier wave to produce a side-band, means for recording the sideband frequencies, means for reproducing the side-band frequencies, ,means for separating the side-band frequencies from any other frequencies produced in said reproducing means,

and demodulating means for restoring the original frequencies.

5. In a system for recording and reproducing sound waves, means for combining the original waves with a carrier Wave to produce a side-band having a lower limit of approximately 5,000 cycles per second, means for separating the side-band from the carrier, means for recording the side-band frequencies, means for reproducing the side-band frequencies, means for separating the side-band frequencies from any other frequencies produced in said reproducing means, and demodulating means for restoring the original frequencies.

6. In a system for recording and reproducing sound wavesvarying infrequency over a band of approximately 10() to 5,000 cycles per second, means for converting the original sound waves into electrical waves, means .for combining said electrical waves with a carrier wave to produce a side-band having a lower limit of approximately 5,000 cycles per second, means for separating the side-band from the carrier, means for recording the side-band frequencies, means for reproducing the side-band frequencies, means for separating the side-band frequencies from an)T other frequencies produced in said reproducing means, demodulating means for restoring the original frequencies, and means for reconverting the resultant waves into sound.

In testimony whereof, I have signed my name to this specification this' 9th day of November, 1927.

CHARLES H. FETTER. 

